2013 DOAS campaign

Summary. The Atmospheric Chemistry and Climate Group
(IQFR, Madrid), led by Alfonso Saiz-Lopez, joins us from Sep 5th to Oct 3rd 2013 at the Pic du Mid for a parallel mercury and halogen survey of the free troposphere. Together with Cristina Prados-Roman and Miguel Fernandez-Sanchez, the team uses the DOAS technique to measure trace amounts of oxidants in the free troposphere. Several of these oxidants such as Br and I radicals potentially oxidize gaseous elemental mercury and the goal of the campaign is therefore to compare the dynamics of mercury and oxidants.

Techniques. LP-DOAS (long path differential optical absorption spectroscopy) uses a Xe lamp that emits in UV, VIS and near-IR wavelengths. The light is focused with a Newtonian telescope on a reflector that we installed 9 kms away on the Pic d'Ayre at 2324m. Target molecules for this campaign were halogens Br, IO, and NO2. LP-DOAS permits day and night measurements.

LP-DOAS instrument in the Antares room at the Pic du Midi

MAX-DOAS (multi axis differential optical absorption spectroscopy) uses reflected sunlight to measure trace amounts of oxidants during day time. The instrument can scan from the horizon to zenith and thereby establsih column profiles into the stratosphere.

Installing the LP-DOAS reflectors. We hired
Mathias Trey, guide de haut montagne, to help us install the three LP-DOAS reflectors on the flank of the Pic d'Ayre at 2324m. Mathias drove us up from "Chez Louisette" in Barreges to the old 'funiculaire' station at 2000m in his 4x4. The final climb was done by foot, with the reflectors mounted on portable racks. Historical images of the early Pic du Midi at the turn of the 20th century came to mind, when 'porteur' was still a genuine profession (see below). The three reflector units, each equiped with 14 corner cubes, were finally installed at 2324m shielded from SW winds and facing the Pic du Midi at a final distance of 9km.

From left to right: Alfonso, Nicolas and Miguel in the 4x4 of mountain guide Mathias Trey

From left to right: Mathias, Alfonso, Miguel carrying the reflectors, and Jeroen.

Sherpa style, with the Pic du Midi in the background (left).

'Porteurs' carrying up the Baillaud telescope in 1906.

Installation of three reflector units, each with 14 corner cubes.

Back at the Pic du Mid: alignment of the Newtonian LP-DOAS telescope on the Pic d'Ayre.

The small white spot in the center of the image on the right flank of the Pic d'Ayre (foreground) is the reflected visible light beam from the LP-DOAS instrument. Photo taken from the Pic du Midi.